Measuring the phase jitter of a high-frequency signal, for example, the output signal of an oscillator, has hitherto generally required a measuring station, which consists of several individual devices, which are relatively expensive and difficult to operate. These are generally a signal generator, a spectrum analyzer for low-frequency Fast-Fourier Transform (FFT), a control computer and a supplementary mixer. To measure the phase jitter, the high-frequency signal to be measured is mixed with a comparison signal of the same frequency generated in the signal generator. The mixer then supplies an output signal, which is the differential frequency between the high-frequency signal to be measured and the comparison signal. This therefore presents a measure for the phase jitter. This differential-frequency signal is evaluated after amplification in a low-noise amplifier using the FFT spectrum analyzer. By mixing both signals to 0 Hz, any AM modulation of the signal that may be present is suppressed, because the two in-phase side bands cancel each other out. In the case of a frequency modulation (FM) of the high-frequency signal to be measured, the side bands are out of phase, and their contribution is therefore added in the mixer.
A known measuring station of this kind for the measurement of phase jitter therefore distinguishes between AM and FM jitter and also allows the measurement of good oscillators. However, it requires a very good low-noise signal generator in the corresponding frequency range. An HF spectrum analyzer is often also required to determine the frequency of the measured signal together with an oscilloscope to check the synchronization. A known measuring station of this kind is therefore relatively complex, expensive and difficult to operate.
In principle, phase jitter can also be measured with a spectrum analyzer. However, the classic spectrum analyzer cannot distinguish between AM and FM modulation, although, generally, only the FM component needs to be measured. With modern spectrum analyzers with digital evaluation of the last intermediate frequency, the AM and FM modulation can in fact be measured separately, but the dynamic range of the analog/digital converter used is not sufficient for the measurement of good low-noise oscillators.